At the data link layer (layer 2 of the Open Systems Interconnection (OSI) model), switching protection protocols, such as the Spanning Tree Protocol (STP), can be used to prevent the formation of loops by blocking node ports. Loops are problematic because they can overwhelm the bandwidth of a network during the flooding process whereby nodes in the network send out requests, which can get repeated ad infinitum in a loop, to find a new destination node with a Media Access Control (MAC) address not previously encountered by a node in the network. When a failure occurs in a network, such switching protection protocols need to be able to respond and restore service.
However, the response time of older switching protection protocols, such as STP, have proved too slow as data-link-layer networks, such as Ethernet networks, have evolved to provide services like video on demand, voice over internet protocol, and internet access. New switching protection protocols have been developed to more quickly respond to failures in the network. Ethernet Ring Protection (ERP) protocols, such as the ERP protocol defined in International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) G. 8032, provide examples of switch protection protocols with dramatically improved recovery times.
Although a node supporting an ERP may have more than two ports into/out of the node, the node can satisfy the ring architecture of an ERP ring by connecting to one or more additional nodes within an ERP ring with two ring ports. Additional ports may connect a node participating in the ring to nodes outside of the ring. Since a ring, by definition results in a loop, ERPs may block a link between a designated pair of nodes to prevent such loops. Despite the blocked link between two nodes, all of the nodes in the ring can still communicate with one another by passing information along the remaining backbone of serviceable links in one direction or another, depending on which is shortest. When a failure occurs elsewhere in the ring, the blocked link can be unblocked, since the failure prevents loops by effectively blocking a new link at a new location. Since the previously blocked link is now unblocked, all the nodes in the ring can still communicate with each other across a new backbone of serviceable links, although the directions in which communications are sent and from which they are received may change do to a new location of a new blocked link.
However, if a node fails completely with respect to the ring, meaning both ring ports go down, even if additional ports to nodes outside the ring remain serviceable, the ring loses two links. With the loss of two links, it is no longer possible to access every node in the ring. Because both ports that connect the failed node to the ring are down, the failed node can no longer communicate with other nodes in the ring and vice versa. What is more, nodes outside the ring connected to the failed node through one or more additional ports, which may not have failed, can no longer communicate with the ring.